As is known, one of the difficulties connected with the extrusion of shapes in thermoplastic material is the regulation of the thrusts in the various points of the die outlet section. Such problem is particularly complicated in the case of plates or, in general, of sections whose one dimension is much greater than the other.
In fact, the longer distance to be covered by the material in order to reach the section ends causes a greater pressure drop and, by consequence, a thrust decrease. This causes different feed speeds of the molten product which, in the case of extrusion of plates, assume very high values.
The aforesaid drawback is generally obviated by having recourse to various contrivances, depending upon the type of shape. In the case of plates extrusion the art suggests, for example, to regulate the thrusts by using a bending bar, located transversally to the head and controlled by a set of adjusting screws, which permits a throttling of the flow section in the points where the molten product flow speed is the higher. By operating in this way and after a suitable adjustment, a regular feed of the plate is achieved.
The above described method, unfortunately, presents several drawback, that may be summarized as follows:
1. Both design and construction of the head are particularly expensive and laborious.
2. Since the bar is subjected to continuous bendings, after a certain time-period it becomes unusable and must be replaced. Moreover, the regulation so attained is rather low.
3. The adjustment operation necessary to attain a regular feed of the plate is often time consuming and complicated resulting in a waste of time and material.
4. The utilization of the bar, although it regulates the thrusts, does not solve the problem connnected with the different run and consequently, the different residence time of the material in the various head zones. This is very detrimental to the proceeding of easily degradable materials (PVC), or -- in general -- of materials, during whose conversion, the amount of heat yielded to the material and the transmission speed of said heat are of particular importance, such as lightened or cellular plastics recently or presently developed.
It is an object of this invention to provide an improved method and apparatus for extruding products into shapes having sections whose one dimension is much larger than the other.
It is also an object of this invention to provide an improved extrusion apparatus for regulating the thrust and flow of speed of molten materials which is simple in design and construction.
It is a further object to provide a method and apparatus for regulating the flow of molten material to provide equal residence time for all material in the extrusion head and providing material of equal temperature and viscosity in all sections of the parts to be extruded. Another object of the present invention is to provide an improved extrusion process and apparatus particularly suited to both flat and corrugated plates or, in general, to shapes having one section much greater than the other.
Finally, it is an object to provide an improved apparatus for controlling the material to be extruded into shapes having a section with one dimension much larger than the other dimension which is readily adapted for use with conventional equipment.